Abstract: A signal recognition circuit (10) is disclosed which is capable of distinguishing between the digit tones used for dialing in multifrequency tone telephone sets, and spurious signals at the same frequencies, such as noise and voice. The discriminator comprises, in cascade, an AGC amplifier (11), an adjustable threshold comparator (12), and a signal validation circuit (13). The operation of the discriminator is based upon the fact that the true dialing tones have envelopes of constant magntiude whereas the spurious signals tend to have pronounced envelope variations. By the appropriate adjustment of the comparator threshold, these variations show up as gaps in the comparator output signal, which are readily detected by the validation circuit.